Revealing the Mechanism of Sodium Diffusion in NaxFePO4 Using an Improved Force Field
Olivine NaFePO4 is a promising cathode material for Na-ion batteries. Intermediate phases such as Na0.66FePO4 govern phase stability during intercalation-deintercalation processes, yet little is known about Na+ diffusion in NaxFePO4 (0 < x < 1). Here we use an advanced simulation technique, Randomized Shell Mass Generalized Shadow Hybrid Monte Carlo Method (RSM-GSHMC) in combination with a specifically developed force field for describing NaxFePO4 over the whole range of sodium compositions, to thoroughly examine Na+ diffusion in this material. We reveal a novel mechanism through which Na+/Fe2+ antisite defect formation halts transport of Na+ in the main diffusion direction , while simultaneously activating diffusion in the  channels. A similar mechanism was reported for Li+ in LiFePO4, suggesting that a transition from one- to two-dimensional diffusion prompted by antisite defect formation is common to olivine structures, in general.